Hi Claire,
I have been keeping a lot of the SPS corals (small polyped stony corals) with my seahorses for quite a few years. It's the stony corals with large polyps (LPS corals) that are usually an issue for seahorses, at least in my experience. So you want to stay away from placing torch corals, frogspawn corals, elegance corals and the like with seahorses. But most acropora and montipora and echinata and chalice corals, etc. are not a problem unless you are keeping dwarf or baby seahorses. If anything, I have found a medium-sized seahorse can be more of a problem for an SPS coral than vice-versa because a seahorse might decide to keep hitching to the same piece of coral and rubbing off the skin and polyps of the unlucky coral, eventually killing it. But one of the ways to avoid this is to offer a lot of alternate hitching posts.
Some of the SPS corals I keep in this tank include red setosa (a type of relatively easy growing montipora), hawkins echinata, ice-fire echinata, PC Rainbow acro, purple bonsai acro, several different types of tenuis acros and millepora acros as well as several chalice corals. You will want to have good flow with the stony corals. I have several small powerheads scattered throughout the tank to achieve this (rather than one or two big ones that might blast the seahorses into a corner. I also use a Tunze mini wave-box which causes the water column in the tank to rock gently back and forth. Just keep in mind that you want to keep the water moving briskly around the tank, but not harsh enough that the seahorses are being shoved into a rock or corner somewhere.
You might want to start off with an SPS coral that is relatively easier to grow, such as a chalice or setosa or monitpora coral. Then try something a little harder once you see that these are doing well. And keep in mind that SPS coral need a lot of Alk and Calcium to thrive. So I dose a relatively small amount of both in my tank by hand everyday. Also, you will want to watch your pH and try to keep it around 8.2 - 8.3 (but if it drops down a bit at night I find that not to be a serious problem).
Whatever you decide to do, I wish you the best! It's nice to know that I am not the only one who thinks that SPS corals and seahorses look good together